Ultrasound transmit and receive path calibration methods and systems

ABSTRACT

The clipping diodes used to limit voltage during medical diagnostic ultrasound imaging operation are used during calibration operation for measuring amplitude and phase differences. The amplitude and phase measurements using clipping diodes as the calibration node may operate at any frequency, including frequencies less than 20 megahertz as well as frequencies above 20 megahertz. By using the diodes already present, a smaller or more compact calibration system may be provided. More simplistic integration may also result.

BACKGROUND

The present invention relates to calibrating transmitters and receiversof an ultrasound imaging system. In particular, amplitude and phaseadjustments for one channel relative to other channels are provided bycalibration.

By measuring amplitude and/or phase differences between differentchannels of an ultrasound imaging system, system induced differences orartifacts may be minimized. For example, transmit wave forms of onechannel are increased in amplitude, decreased in amplitude, or phaserotated to account for system induced differences relative to anotherchannel. Likewise, receive performance may also be calibrated.Calibration may allow for images with less system induced noise.Calibration ascertains output amplitude and phase shifts betweentransmitters, or ascertains voltage gain and phase shift betweenreceivers.

Hardware is positioned within the imaging system adjacent to atransducer port for calibration. The coupling node for calibration isused to inject signals of well-defined properties for receivercalibration and precisely monitor signals for transmit calibration. FIG.1 shows one embodiment of a calibration system 10 for ultrasoundimaging. A plurality of transmitters 12 and receivers 14 connect withrespective transmit and receive lines 16. For use in imaging, pairs ofdiodes 20 connect to respective transmit and receive lines 16. One diode20 of each pair connects with a high voltage source, and the other diode20 of each pair connects with a negative voltage source. The positiveand negative voltage sources act to limit a possible voltage output oneach transmit and receive line to avoid injury.

A calibration node 18 also connects with each of the transmit andreceive lines 16. The calibration node 18 is used for calibrating priorto imaging. The calibration node 18 includes resistive or capacitivecomponents. For example, each transmit and receive line 16 is connectedtogether through low valued capacitors (e.g., less then 50 picofarads)to a common node or conductor. For example, a printed wiring board traceis run under the transmit and receive lines to form the capacitivecommon node. The common node is connected to ground through a largercapacitor (e.g., greater than 1,000 picofarads) in order to limit theamount of unwanted crosstalk introduced from line-to-line capacitance.For calibrating receivers, a signal is generated on the common node, andeach transmit and receive lines 16 is measured sequentially using thereceivers 14. Amplitude and phase differences are identified from themeasured signal. For transmit calibration, the transmitters 12sequentially generate transmit waveforms on the transmit and receiveline 16, and an amplitude and phase is measured at the common node.However, this capacitive calibration node may not be suitable forsystems operating at high frequencies, such as greater than 20 MHz. Asthe frequency increases, the impedance of a capacitor decreases. Thecapacitive impedance to ground decreases within the calibration node 18,which may cause discontinuity in the transmission lines, likelydegrading system performance.

In another approach, the calibration node 18 is a resistive device. Eachof the transmit and receive lines 16 are connected together throughresistors, such as on the order of less than 1 kilo-ohms. The commonnode is then connected to ground through a relatively small valuedresistor, such as less than 100 ohms, in order to limit the amount ofunwanted crosstalk introduced from line-to-line resistance. The commonnode is used as discussed above to determine relative amplitude andphases for both transmit and receive operation. In normal imagingoperation, the resistive based calibration node 18 may operate over awide range of frequency signals, including greater than 20 megahertz.However, in calibration mode, as the frequency increases, the resistorsincreasingly act as low pass filters because of stray capacitance toground. Degraded calibration performance may result. Substantialtransmit power loss over all frequencies may also result.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods and systems for calibration of an ultrasound transmitterand/or receiver. The clipping diodes used to limit voltage duringimaging operation may be used during calibration operation for measuringamplitude and phase differences. Using clipping diodes as thecalibration node may allow operation at any frequency, includingfrequencies less than 20 megahertz as well as frequencies above 20megahertz. By using the diodes already present for imaging reasons, asmaller or more compact calibration system may be provided. Moresimplistic integration may also result.

In a first aspect, a system for calibration of an ultrasound transmitterand/or receiver is provided. A clipping diode connects with anultrasound transmit, receive or transmit/receive path. A voltage sourceis connectable with the clipping diode. A switch is between the clippingdiode and the first voltage source for selecting between calibration andnormal modes of operation.

In a second aspect, a system for calibration of an ultrasoundtransmitter and receiver is provided. Pairs of clipping diodes connectwith respective ones of transmit/receive ultrasound paths. A positivevoltage source connects with one of each pair of the clipping diodes,and a negative voltage source connects with another of each pair of theclipping diodes. A switch is provided between the first ones of theclipping diodes and the positive voltage source. Another switch isconnected between the second ones of the clipping diodes and a negativevoltage source. The switches are operable to select between calibrationand imaging modes of operation.

In a third aspect, a method is provided for calibration of an ultrasoundtransmitter and/or receiver. At least one of phase and amplitude aremeasured with a signal provided through a clipping diode. The ultrasoundtransmitter and/or receiver are calibrated as a function of the phase oramplitude information.

In a fourth aspect, an improvement in a method for calibrating atransmit/receive path of an ultrasound system where clipping diodeslimit an output voltage of the transmit/receive path is provided. Theimprovement includes using the clipping diodes as a calibration node forthe transmit/receive path.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below in theDetailed Description of the Drawings and Presently PreferredEmbodiments. The aspects and advantages discussed within this Summary aswell as the further aspects and advantages may be now claimed or laterclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a circuit diagram of a prior art calibration system forultrasound transmitters and receivers;

FIG. 2 is a circuit diagram showing one embodiment of an ultrasoundtransmitter calibration system using clipping diodes;

FIG. 3 is a circuit diagram of one embodiment of an ultrasound receivercalibration system using clipping diodes;

FIG. 4 is a flow chart diagram of one embodiment of a method forcalibrating an ultrasound system; and

FIG. 5 is a circuit diagram showing one embodiment of a transmitter andreceiver calibration system using clipping diodes.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

For safety reasons, transmit lines or transmit/receive lines inultrasound medical diagnostic imaging systems are connected to positiveand negative high voltage sources with clipping diodes. In the event ofa system fault, the clipping diodes ensure that the output voltage maynot exceed safety limits. The same clipping diodes may be used forcalibration. In imaging operation, the clipping diodes are connected tothe high voltage sources. For calibration operation, the clipping diodesare connected to a signal source or the calibration receiver. Using thecommon connection of the clipping diodes as a calibration node mayreduce cost and board area since capacitors and resistors of acapacitive or resistive calibration node may be eliminated. Duringimaging operation, the calibration node may not introduce adiscontinuity in the transmission lines, allowing for higher frequencyoperation.

FIG. 2 shows a system for calibration of an ultrasound transmitter inone embodiment. FIG. 3 shows a system for calibration of an ultrasoundreceiver in one embodiment. While shown separately, the systems may becombined to provide a system for calibration of ultrasound transmittersand receivers as shown in FIG. 5. The combination is provided byconnecting the current source 36 to the input of the amplifier 32 andoperating the current source 36 and the amplifier 32 at different times.Alternatively, a switch or other device is provided to switch betweencalibration of the transmitters and its associated structure andcalibration of the receiver and its associated structure. As yet anotheralternative, the switches 26 and 28 are three-pole switches forswitching between the imaging operation, transmit calibration andreceive calibration.

The systems of FIGS. 2 and 3 include transmitters 12, receivers 14,transmit/receive lines 16, clipping diodes 20, a positive high voltagesource 22, a negative high voltage source 24, and switches 26, 28.Additional, different and fewer components may be provided, such asproviding transmit lines 16 with transmitters without a receiver 14. Asanother example, the receiver 14 is provided with a receive line 16without the transmitter 12. As yet another example, a multiplexer orpartial beamforming for combining data from a plurality of transducerelements onto a single transmit or receive line 16 is provided.

The transmitter 12 is an amplifier, analog-to-digital converter,switches, transistors, filters, delays, waveform generator or other nowknown or later developed device for generating unipolar, bipolar orsinusoidal waveforms, such as a transmit beamformer channel. Forexample, the transmitter 12 is a transistor network for generatingunipolar or bipolar square waves. Alternatively, the transmitter 12 is amemory, phase rotators, filters, amplifiers and other structuresdisclosed in U.S. Pat. No. 5,675,554, the disclosure of which isincorporated herein by reference, for generating sinusoidal waveforms.

The receiver 14 is a filter, delay, phase rotator, mixer, summer,amplifier or other now known or later developed ultrasound receiver,such as an ultrasound receive beamformer channel. The receiver 14connects with the transmitter 12 through a transmit/receive switchconnected to the transmit/receive lines 16. Alternatively, other formsof protecting the receiver 14 from the output of the transmitter 12 areprovided.

The transmit/receive lines 16 are conductive lines, such as traces on acircuit board, a flexible circuit, a bus, coaxial cables, or otherconductive structures now known or later developed. Each of thetransmit/receive lines 16 operates as separate ultrasound path. Forphased array beamforming, a plurality of the transmit/receive line 16are used for focusing along one or more scan lines in connection with arespective plurality of transducer elements. For transmission, thetransmit/receive lines 16 acts as a transmit path from the transmitter12 to an associated transducer element. For receive operations, thetransmit/receive line 16 operates as a receive path from the transducerelement to the associated receiver 14. In alternative embodiments,separate transmit and receive paths are provided. None or one or moreintervening components may be provided along the transmit/receive line16, such as a preamplifier, multiplexer, transmit/receive switch, filteror other structures. In one embodiment, the transmit/receive line 16 isa path between an amplifier and the transducer, such as between anapodization amplifier of the transmitter 12 or an apodization amplifieror preamplifier of the receiver 14 and an associated transducer element.

A pair of clipping diodes 20 connects with each of the transmit/receivelines 16. The clipping diodes 20 are zener diodes, transistor baseddiodes or any now known or later developed diodes. One diode 20 isconnected with the transmit/receive line 16 to prevent current frompassing until a voltage on the transmit/receive line reaches a highpositive voltage. Conversely, another of the pair of diodes 20 isconnected to the transmit/receive line 16 to prevent the flow of currentthrough the diode 20 until the voltage on the transmit/receive line 16drops to a high negative voltage. The pairs of clipping diodes 20 forthe transmit/receive lines 16 are connected together in parallel asshown in FIGS. 2 and 3. Additional, different or fewer diodes 20 may beused. During imaging operation, the pairs of diodes 20 connected withrespective ones of the transmit/receive lines 16 act as clipping diodesto prevent a voltage on the transmit/receive line from exceeding thehigh positive voltage or high negative voltage.

The high positive voltage and high negative voltage are set by thepositive high voltage source 22 and the negative high voltage source 24.In one embodiment, the high voltage sources are 200 and −200 volts, butother voltage values may be used. The high voltage sources 22, 24 aretransformers, voltage dividers or other now known or later developedvoltage sources. In one embodiment, each of the voltage sources 22, 24is a DC voltage source, but an AC component may be provided. Each of thevoltage sources 22, 24 is connectable with one or more of the clippingdiodes 20. For example, the positive high voltage source 22 connectswith one of each pair of clipping diodes 20, and the negative highvoltage source 24 connects with the other of the clipping diodes 20 ofeach pair.

The switches 26 and 28 are relays, transistors or other now known orlater developed switching devices. Each switch 26, 28 is between one ormore clipping diodes 20 and one of the voltage sources 22, 24. Forexample, the switch 26 is positioned between the high voltage source 22and half of the clipping diodes 20 or one clipping diode 20 for eachtransmit/receive line 16. The switch 28 is positioned between thenegative high voltage source 24 and the other half of the clippingdiodes 20 or one clipping diode 20 for each transmit/receive line 16. Inthe configurations shown in FIGS. 2 and 3, the switches 26, 28 aredouble pole relay switches for selecting between an imaging mode ofoperation through connection to the voltage sources 22 and 24 and acalibration mode of operation by connection to either of the transmit orreceive calibration circuitry. In alternative embodiments, a triple polerelay or other switch is provided for selecting between imagingoperation, receive calibration and transmit calibration. Additionalpoles or additional switches may be provided for implementing otherfunctions or selectable connection of other circuitry.

During imaging operation, the switch 26 connects the positive voltageclipping diodes 20 to the positive high voltage source 22. The switch 28connects the negative clipping diodes 20 to the negative high voltagesource 24. Where a fault or other error occurs resulting in a voltage onone or more of the transmit/receive lines, any current and associatedvoltage in excess of the negative or positive high voltages is drainedto the voltage source. As a result, the clipping diodes 20 act to limitthe voltage on the transmit/receive line 16 to prevent injury to a useror a patient.

For calibration, the switches 26 and 28 connect the clipping diodes 20to the transmit calibration circuit 29 and receive calibration circuit35 or both the transmit and receive calibration circuits 29, 35. Thetransmit calibration circuit 29 includes an analog-to-digital converter30, an amplifier 32 and a resistor 34. Additional, different or fewercomponents may be provided. For example, an analog-to-digital converter30 is provided without the amplifier 32. Other devices now known orlater developed for measuring an amplitude or phase of a signal may beused as the transmit calibration circuit 29.

In one embodiment, the resistor is a 0.33 ohm resistor, but other valueresistors may be used depending on the components. For example, thetransmitters 12 are operable to provide one to two amps of current.During calibration, all of the transmitters or only one of thetransmitters 12 is connected to a single point or calibration node. Inthe calibration mode, the clipping diodes 20 act as anti-parallel diodesto the calibration node. The calibration node connects with the transmitcalibration circuit 29 and is provided through the clipping diodes 20.Each transmitter 12 is sequentially excited to produce a transmitwaveform. For a maximum of one to two amps, about a 0.66 positive ornegative voltage is generated across a 0.33 ohm resistor.

The amplifier 32 is a feedback amplifier, differential amplifier,transistors or any other now known or later developed amplifier forscaling or increasing the value of the voltage at the resistor 34. Theinput of the amplifier 32 connects with each of the switches 26 and 28when closed for calibration mode operation and with the resistor 34. Theresistor 34 connects the input of the amplifier 32 to ground, such as arelative or absolute ground.

The analog-to-digital converter 30 is a digital circuit, analog circuit,combinations thereof or any other now known or later developedanalog-to-digital converter. The analog-to-digital converter 30 connectswith the switches 26 and 28 through the amplifier 32. In response to theoutput of the amplifier 32, the analog-to-digital converter 30 generatesdigital values representing the analog waveform received from atransmitter 12 through the calibration node for calibration. Amicroprocessor or other circuit is then used to identify a maximumamplitude, peak positive amplitude, peak negative amplitude and/orrelative phasing. By comparing the amplitudes and phase information foreach of the transmitters 12, a relative phase and/or amplitudeadjustment of one transmitter 12 to other transmitters 12 is determined.The relative adjustment is implemented in the amplifiers and delays orphase rotators of the transmitter 12 so that more uniform transmitwaveforms are generated by each of the transmitters 12.

Referring to FIG. 3, the receive calibration circuit 35 includes acurrent source 36. Additional or different components may be provided.The current source 36 is a transistor, amplifier, integrated circuit,operational amplifier, passive components, combinations thereof or othernow known or later developed current source 36. The current source 36connects with the calibration node through the switches 26 and 28. Usingthe switches 26 and 28, the clipping diodes 20 act as anti-paralleldiodes. In alternative embodiments, one of the switches 26, 28 is notconnected to the current source 36 during calibration operation. Thecurrent source 36 is operable to provide a DC or an AC current. Forexample, the current source 36 is a transistor, amplifier or otherstructure for generating a waveform emulating likely waveforms receivedover the transmit/receive lines 16. For example, a sinusoidal waveformresulting in a voltage less than 0.7 volts positive or −0.7 volts isgenerated. The current passes through the switches 26 and 28 and theclipping diodes 20 to the transmit/receive lines 16. Using sequentialoperation, each receiver 14 is sequentially operated to measure theamplitude and phase of the received waveform. For example, the receivers14 digitize the analog waveforms with analog-to-digital converters andmeasure the gain and phase using processors. A relative gain and phaseof each receiver 14 as compared to other receivers 14 is used todetermine an adjustment. As a result, differences in the receivers 14are minimized or calibrated so that waveforms received at eachtransducer element are beamformed without artifacts introduced by thereceivers 14. Any gain or phase adjustments are then applied by theapodization amplifiers and phase rotators or delays of the receivers 14during imaging.

FIG. 5 shows one embodiment of a circuit for both transmit and receivecalibration of an ultrasound system using clipping diodes. The switches26 and 28 operate as described above, but connect to different transmitand receive calibration circuitry.

The switch 26 selectably connects between the positive high voltagesource 22 and a current source 40. The current source 40 is atransistor, amplifier, integrated circuit, operational amplifier,passive components, combinations thereof or other now known or laterdeveloped current source. In one embodiment, the current source has anoutput impedance of about or at least 1000 Ohms and sinks about 1 mA foreach transmit/receive path 16. For example, if there are thirty twotransmit/receive paths 16 connected through the clipping diodes 20 tothe same switches 26, 28 and/or current source 40, the current source isoperable to sink 32 mA.

The switch 28 is connected to another current source 42. The currentsource 42 is a transistor, amplifier, integrated circuit, operationalamplifier, passive components, combinations thereof or other now knownor later developed current source. The current source 42 has similarimpedance characteristics as discussed above for the other currentsource 40, but may have different impedance characteristics. The currentsource 42 is operable to provide a same level of current as sunk by theother current source 40, but a lesser or greater amount may be provided.The level of current provided is controlled, preset or provided as afunction of the amount of current drawn.

For calibration, the switches 26 and 28 connect to the current sources40 and 42. This connection forward biases the clipping diodes 20 andswitches the clipping diodes 20 into a low impedance state. Thetransmit/receive paths 16 are more effectively connected to a commontransmit and receive calibration node 44. For transmit calibration, theamplifier 32 and analog-to-digital converter 30 operate as describedabove. At a different time, receive calibration is provided by injectingsignals from the current source 36 as described above.

The transmitters 12 may include one or more grass clipper diodes toelectrically isolate the transmitters 12 from the transmit/receive path16 during receive operation or receiving small electrical signals.Similarly, each receiver 14 may include a switch to isolate the receiver14 from the transmit/receive path 16 during high voltage transmitoperation. The individual transmitters 12 and receivers 14 arecalibrated with a single or the common transmit and receive calibrationnode 44 for calibrating multiple channels in one embodiment.Alternatively, separate calibration circuitry is provided for differentones or groups of transmitters 12 and/or receivers 14. Duringcalibration, the transducer elements are isolated from thetransmit/receive paths 16. For example, any transducer is disconnected.Alternatively, switches are used to isolate the transducer elements. Asyet another alternative, the system switches the transmit/receive paths16 for calibration to connect with a transducer connector that is notcurrently used.

In a method for calibrating the transmit/receive path of an ultrasoundsystem where the transmit/receive path includes clipping diodes to limitan output voltage, the clipping diodes are used as a calibration nodefor the transmit/receive path. Using the clipping diodes as thecalibration node may avoid or minimize connection of resistors orcapacitances to the transmit/receive line. The current used forcalibrating in both transmit and receive passes through the clippingdiodes 20. Using the clipping diodes as the calibration node may reducethe costs and circuit board area dedicated to calibration. Discontinuityon the transmission lines is avoided during normal operation, allowingfor higher frequency operation in imaging. Calibration at higherfrequencies is also provided without degradation of performance.

FIG. 4 shows one embodiment of a method for calibration of ultrasoundtransmitter and/or receivers. Additional, different or fewer acts may beprovided in the same or different order. For example, an imaging mode ofact 50 is implemented prior to the calibration mode of act 42.

In act 42, a calibration mode is implemented. Any dual functioncomponents are switched to a calibration function. For example, clippingdiodes are disconnected from high voltage sources and connected totransmit or receive calibration circuits. Measurement and calibrationare performed while the clipping diodes are disconnected from the highvoltage sources. One or more clipping diodes may alternatively beconnected to a voltage source while other clipping diodes are used forcalibration. When the clipping diodes are not currently connected to thevoltage source, the clipping diodes are disconnected from the highvoltage source without further switching. The transmitters or receiversare configured for calibration, such as controlling the transmitters andreceivers to operate in the sequential manner as a function of path orbeamformer channel.

In act 44, a calibration signal is generated. For transmit calibration,a transmit waveform is generated on a transmit path. In response to thetransmit waveform on a transmit path, a signal is generated through aclipping diode. The signal acts as a transmit calibration signal. Eachof the transmitters or transmit paths sequentially generates acalibration signal and associated measurement of the signal. In oneembodiment, all of the transmit paths, such as 64, 128, 196, 256 orother numbers of transmit paths are sequentially used to generatecalibration signals.

For receive calibration, a signal is generated with a current sourceconnected with the clipping diodes. The current source generates asignal on one, all or a subset of the receive paths. Each of thereceivers associated with the receive path is sequentially operated toreceive the generated receive calibration signal. In an alternativeembodiment, groups or all of the receivers are operated substantiallysimultaneously to receive the generated calibration signal.

In act 46, the calibration signal is measured. For transmit calibration,a transmit calibration circuit measures the calibration signal. Forreceive calibration, receivers of the receive beamformer measure thecalibration signal. One or both of phase and amplitude are measured. Themeasured calibration signal is provided through one or more clippingdiodes operated as anti-parallel diodes for calibration. For eachtransmit/receive line, the calibration signal may be responsive tomultiple clipping diodes, such as the pair of diodes connected with thetransmit/receive line. The measurement is performed in one embodiment byconverting the calibration signal from an analog-to-digital form.Alternatively, measurements are made on an analog waveform withoutconversion. A peak amplitude, a peak positive amplitude, a peak negativeamplitude, and/or phase are measured. In one embodiment, the offset ofthe measured amplitudes and phase from the generated waveform isdetermined. Each of the transmitters or receivers is adjusted as afunction of the offset from the source of the calibration signal. Inother embodiments, the offsets are relative by comparing amplitudes andphases between different channels without comparison to the generatedcalibration signal.

In act 48, an ultrasound transmitter and/or receiver is calibrated as afunction of the phase, amplitude or combinations thereof. For example, aphase, amplitude or both phase and amplitude characteristic of atransmit path is adjusted relative to a different transmit path. Theamount of delay or phase rotation and amount of apodization is adjustedduring imaging operation. As another example, the phase, amplitude orboth the phase and amplitude characteristic of one received path isadjusted relative to another receive path. During imaging operation, theadjustment is implemented as a change of gain, delay or phase rotationfor receiving ultrasound signals. The calibration may result in a moreuniform operation as a function of channel by the transmit or receivebeamformers. System induced differences are minimized.

In act 50, an imaging mode is implemented. The clipping diodes areconnected with high voltages. The transmit/receive paths are thenoperated while the clipping diodes are connected with the high voltages.The clipping diodes and the high voltages act to limit an output voltageof an ultrasound transmitter. The output voltage is limited betweenpositive and negative voltages connected through the pairs of clippingdiodes. The connection and disconnection of the clipping diodes from thehigh voltage sources and the calibration circuits provides for operationin imaging and calibration modes. During the imaging mode, thetransmitters 12 transmit waveforms generated as a function ofcalibration adjustments. For receive operations, the receivers 14receive and process signals representing acoustic echoes as a functionof calibration adjustments.

In one embodiment, each transmit/receive line 16 is associated with asingle transducer element. In other embodiments, each transmit/receiveline 16 is associated with two or more transducer elements. For example,the signals from two or more elements are multiplexed onto a sametransmit/receive line 16. To minimize loss of information, theassociated frequency of signals along the transmit/receive line 16 isincreased. For the single transducer embodiment, a higher imagingfrequency may be used. Regardless of the source, using the clippingdiodes as a calibration node may avoid resistive or capacitive baseddiscontinuities or degradation provided by capacitive or resistivecalibration nodes for higher frequency imaging or calibration.

The calibration is performed at the place of manufacture in oneembodiment. All subsequent imaging is then performed in response to thesame calibration. Alternatively or additionally, the calibration isperformed in response to other triggers. For example, calibration isautomatically performed each time a new transducer is connected with animaging system, every certain period of time, during routinemaintenance, at the beginning of each imaging session, in response to abroadcast trigger or periodically within an imaging session. Thecalibration is performed while the transducer is spaced away from apatient.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents that are intended todefine the spirit and scope of this invention.

1. A system for calibration of an ultrasound transmitter and/orreceiver, the system comprising: at least one of an ultrasound transmitand receive path; a first clipping diode connected with the path; afirst voltage source connectable with the first clipping diode; and afirst switch between the first clipping diode and the first voltagesource.
 2. The system of claim 1 wherein the at least one of theultrasound transmit and receive path comprises an ultrasound transmitand receiver path.
 3. The system of claim 1 wherein the path comprises aconductor connected between an amplifier and a transducer.
 4. The systemof claim 1 wherein the first voltage source comprises a positive voltagesource; further comprising: a second clipping diode connected with thepath; a second voltage source comprising a negative voltage source, thesecond voltage source connectable with the second clipping diode; and asecond switch between the second clipping diode and the second voltagesource.
 5. The system of claim 1 further comprising: a plurality ofseparate ultrasound paths including the at least one path; andadditional clipping diodes connectable in parallel with the firstvoltage source and respective ones of the plurality of separateultrasound paths; wherein the first switch connects with each of theadditional clipping diodes.
 6. The system of claim 1 wherein the firstswitch comprises a double pole relay.
 7. The system of claim 1 whereinthe path comprises a transmit path; further comprising: ananalog-to-digital converter connected with the first switch, the firstswitch operable to select between connecting the first clipping diodewith the first voltage source for transmit operation and with theanalog-to-digital converter for calibration operation.
 8. The system ofclaim I wherein the path comprises a receive path; further comprising: acurrent source connected with the first switch, the first switchoperable to select between connecting the first clipping diode with thefirst voltage source for receive operation and with the current sourcefor calibration operation.
 9. A system for calibration of an ultrasoundtransmitter and receiver, the system comprising: a plurality oftransmit/receive ultrasound paths; a plurality of pairs of clippingdiodes, each of the pairs of clipping diodes connected with a respectiveone of the transmit/receive ultrasound paths; a positive voltage sourceconnectable with a first one of the clipping diodes of each pair ofclipping diodes; a negative voltage source connectable with a second oneof the clipping diodes of each pair of clipping diodes; a first switchbetween the first ones of the clipping diodes and the positive voltagesource; and a second switch between the second ones of the clippingdiodes and the negative voltage source.
 10. The system of claim 9further comprising: an amplifier having an input connectable with thefirst and second switches; a resistor connected between the input and aground; and an analog-to-digital converter connected with an output ofthe amplifier.
 11. The system of claim 9 further comprising: a currentsource connectable with the first and second switches.
 12. A method forcalibration of an ultrasound transmitter and/or receiver, the methodcomprising: (a) measuring at least one of phase and amplitude with asignal provided through a clipping diode; and (b) calibrating theultrasound transmitter and/or receiver as a function of the at least oneof phase and amplitude.
 13. The method of claim 12 further comprising:(c) connecting the clipping diode with a high voltage; (d) operating atransmit/receive path connected with the clipping diode in an imagingmode while the clipping diode is connected with the high voltage; and(e) disconnecting the clipping diode from the high voltage; wherein (a)and (b) are performed while the clipping diode is disconnected.
 14. Themethod of claim 12 further comprising: (c) limiting an output voltage ofthe ultrasound transmitter with the clipping diode.
 15. The method ofclaim 14 wherein (c) comprises limiting the output voltage between apositive and negative voltage with the clipping diode and an additionalclipping diode, wherein (a) the at least one of phase and amplitude isresponsive to the signal through the clipping diode and an additionalsignal through the additional clipping diode.
 16. The method of claim 12further comprising: (c) generating a transmit waveform on a transmitpath, the signal through the clipping diode responsive to the transmitwaveform.
 17. The method of claim 16 wherein (a) and (c) are performedsequentially for each of at least two different transmit paths, wherein(b) comprises adjusting one of a phase and amplitude characteristic ofone of the two different transmit paths relative to the other of the twodifferent transmit paths during imaging operation.
 17. The method ofclaim 12 wherein (a) comprises converting the signal fromanalog-to-digital.
 18. The method of claim 12 further comprising: (c)generating the signal with a current source connected with the clippingdiode; wherein (a) comprises measuring the signal on an ultrasoundreceive path.
 19. The method of claim 18 wherein (a) and (c) areperformed sequentially for each of at least two different receive paths,wherein (b) comprises adjusting one of a phase and amplitudecharacteristic of one of the two different receive paths relative to theother of the two different receive paths during imaging operation. 20.In a method for calibrating a transmit/receive path of an ultrasoundsystem wherein clipping diodes limit an output voltage of thetransmit/receive path, an improvement comprising: using the clippingdiodes as a calibration node for the transmit/receive path.
 21. Thesystem of claim 4 further comprising: first and second current sources;wherein the first switch selectably connects the first clipping diodewith one of the first current source and the first voltage source andwherein the second switch selectably connects the second clipping diodewith one of the second current source and the second voltage source. 22.The system of claim 9 further comprising: first and second currentsources; a transmit calibration circuit connected to a common node; areceive calibration circuit connected to the common node; wherein thefirst switch selectably connects the first one of the clipping diodeswith one of the first current source and the positive voltage source;and wherein the second switch selectably connects the second one of theclipping diode with one of the second current source and the negativevoltage source, the second switch connectable with the common node.